As costs of energy increase along with concerns about global warming due to consumption of fossil fuels to generate energy, there is an every increasing need for more efficient lighting technologies. These demands, coupled with rapid improvements in semiconductors and related manufacturing technologies, are driving a trend in the lighting industry toward the use of light emitting diodes (LEDs) or other solid state light sources to produce light for general lighting applications, as replacements for incandescent lighting and eventually as replacements for other older less efficient light sources.
The actual solid state light sources, however, produce light of specific limited spectral characteristics. To obtain white light of a desired characteristic and/or other desirable light colors, one approach uses sources that produce light of two or more different colors or wavelengths and one or more optical processing elements to combine or mix the light of the various wavelengths to produce the desired characteristic in the output light. In recent years, techniques have also been developed to shift or enhance the characteristics of light generated by solid state sources using phosphors, including for generating white light using LEDs.
Phosphor based techniques for generating white light from LEDs, currently favored by LED manufacturers, include UV or Blue LED pumped phosphors or nanophosphors. The phosphor materials may be provided as part of the LED package (on or in close proximity to the actual semiconductor chip), or the phosphor materials may be provided remotely (e.g. on or in association with a macro optical processing element such as a diffuser or reflector outside the LED package). The remote phosphor based solutions have advantages, for example, in that the color characteristics of the fixture output are more repeatable, whereas solutions using sets of different color LEDs and/or lighting systems with the phosphors inside the LED packages tend to vary somewhat in light output color from fixture to fixture, due to differences in the light output properties of different sets of LEDs (due to lax manufacturing tolerances of the LEDs).
Although these solid state lighting technologies have advanced considerably in recent years, there is still room for further improvement. For example, it is desirable in the lighting industry to provide lighting systems, which when installed, blend in or are neutral with their surrounding environments, such as ceilings, which are typically white in color. An installed lighting system is more visibly pleasing when its overall observed color is white or silver. However, when certain remote phosphor materials are used in lighting systems, they are often visible from outside of the fixture when not in use. Some phosphor materials for example, may have an undesirable salmon or yellowish color.